Thermal dye sublimation transfer also called thermal dye diffusion transfer is a recording method in which a dye-donor element provided with a dye layer containing sublimable dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, heated with a thermal printing head provided with a plurality of juxtaposed heat-generating resistors, whereby dye from the selectively heated regions of the dye-donor element is transferred to the receiver sheet and forms a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which is covered with a dye layer, which contains the printing dyes. Usually an adhesive or subbing layer is provided between the support and the dye layer.
Due to the fact that the thin support softens when heated during the printing operation and then sticks to the thermal printing head thereby causing malfunctioning of the printing apparatus and reduction in image quality the backside of the support (side opposite to the dye layer) is typically provided with a heat-resistant layer to facilitate passage of the dye-donor element under the thermal printing head. An adhesive layer may be provided between the support and the heat-resistant layer.
The heat-resistant layer generally comprises a lubricating material and a binder. In the conventional heat-resistant layers the binder is either a cured binder (as described in, for example, EP 153880, EP 194106, EP 314348, EP 329117, JP 60/151096, JP 60/229787, JP 60/229792, JP 60/229795, JP 62/48589, JP 62/212192, JP 62/259889, JP 01/5884, JP 01/56587, JP 02/128899) or a polymeric thermoplast (as described in, for example, EP 267469, JP 58/187396, JP 63/191678, JP 63/191679, JP 01/234292, JP 02/70485).
These thermostable binders are usually mixed with lubricants such as silicones, fluorine-containing compounds and the like. When these lubricating agents are incorporated in the heat-resistant layer, only a small portion of the lubricating material is in direct contact with the thermal head leading to ineffective slipping relative to the thermal head during printing and occurrence of color drift. This problem can be solved by applying the lubricating material as a separate topcoat on top of the heat-resistant polymeric layer. Such an assembly of a heat-resistant layer and a separate lubricating topcoat on top of the heat-resistant layer is described in U.S. Pat. No. 4,666,320.
Silicone-based lubricants, such as liquid silicone oils, and liquid silicone blockcopolymers (e.g. blockcopolymers of polysiloxane and polyether) can be used as separate topcoats on top of the heat-resistant layer. This improves the slipping properties of the donor element. However, dyes in the dye donor element tend to crystallize during storage of the dye-donor element in rolled form, due to the contact between the silicone compound contained in the topcoat of one wrapping and the dye donor layer of the underlying wrapping. This problem can be solved by using solid silicones or crosslinked silicones in the topcoat layer. However, the slipping properties of a donor element with such a topcoat are insufficient and color drift occurs during printing.
Inorganic polymers have been described as slipping layers in contact with the thermal head (see U.S. Pat. No. 4,764,496), however, without the addition of slipping agents such as silicones. These slipping layers tend to stick to the thermal head, causing malfunctioning of the printing apparatus.